One of the biggest problems caused from vehicles using fossil fuel, such as gasoline and diesel oil, is the creation of air pollution. A technology of using a secondary battery, which can be charged and discharged, as a power source for vehicles has attracted considerable attention as one method of solving the above-mentioned problem. As a result, electric vehicles (EV), which are operated using only a battery, and hybrid electric vehicles (HEV), which jointly use a battery and a conventional engine, have been developed. Some of the electric vehicles and the hybrid electric vehicles are now being commercially used. A nickel-metal hydride (Ni-MH) secondary battery has been mainly used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). In recent years, however, the use of a lithium-ion secondary battery has been attempted.
High power and large capacity are needed for such a secondary battery to be used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). To this end, a plurality of small-sized secondary batteries (unit cells) is connected in series to each other so as to form a battery module. According to circumstances, the small-sized secondary batteries (unit cells) are connected in series and in parallel to each other so as to form a battery module.
Generally, such a battery module has a structure to protect unit modules, each of which has secondary batteries mounted therein. The structure of the battery module may be various based on the kind of vehicles or installation position of the battery module in the vehicles. One of the structures to effectively fix large-capacity unit modules is based on supporting bars and end plates. This structure has an advantage in that movement of the unit modules is minimized even when load is applied to the supporting bars. To this end, however, it is necessary to sufficiently secure the strength of the supporting bars and the end plates.
In particular, for an end plate located perpendicularly to the direction in which load is applied, bending load generated due to weight of the unit modules is applied to the end plate. For this reason, the end plate must have a structure to properly distribute the bending load. If the end plate does not have a structure to properly distribute the bending load, a weak region of the end plate, such as a coupling region of the end plate or a corner of the end plate, may be severely damaged.
For example, in a structure in which corners 33 are open as in an end plate 30 of FIG. 2, the end plate 30 does not serve to support bending at all. When weight of the supporting bars and the unit modules is applied to the end plate 30 of FIG. 2, therefore, the end plate 30 is severely deformed, with the result that bent portions of the end plate 30 are damaged.
In a structure in which corners 35 of the end plate 30a are integrally connected as in an end plate 30a of FIG. 3 so as to complement the end plate 30 of FIG. 2, damage to bent portions of the end plate 30a does not occur unlike the structure of FIG. 2. However, bending load concentrates on coupling regions 37 of the end plate 30a, with the result that the coupling regions 37 are damaged.
Also, when the battery module is installed in a trunk of a vehicle, a portion of the base plate is mounted above a region where a spare tire is located due to the layout of the vehicle. That is, the battery module is installed in an asymmetrical structure. When vibration from a road surface is severe, twisting load is applied to the battery module, and the twisting load is transmitted to the end plate, with the result that the end plate may be easily damaged.
Therefore, there is a high necessity for a vertical stack type battery module having a structure to stably maintain a stacked structure of unit modules and properly disperse bending load such that bent portions or coupling regions of end plates are not damaged even when pressure from supporting bars and unit modules are applied to end plates.